The present invention pertains to testing elements for the colorimetric determination of oxidizable gas and/or vapor components in gas mixtures. The testing elements are usually designed as detector tubes, which contain carrier materials, to which at least one oxidizing agent is applied. The present invention also pertains to redox indicators that can be used in the testing elements, especially special benzidine derivatives.
Various designs of colorimetric measuring systems, by means of which gases can be tested for components contained in them simply, rapidly and inexpensively, have been known. A device available as Alco-Check(trademark) for the semidquantitative determination of the blood alcohol concentration from the alcohol content in the exhaled air is a known example. The breathing air is blown here through a detector tube containing chromosulfuiric acid on SiO2. If the breathing air contains alcohol vapor, the color of the originally yellow substance of the detector tube changes to green as a consequence of the reduction into a chromium(III) compound. The lower detection limit is 0.3 promille.
As is shown by the above example, colorimetric measuring systems for gases usually comprise testing elements, preferably detector tubes, which contain reagents that are adsorbed on carrier substances and form colored reaction products with the gaseous or vapor substance to be tested, optionally by means of a catalyst. The intensity and the length of the colored zone make possible a qualitative or quantitative evaluation.
The automatic optoelectronic evaluation of the change in the color of the detector tube has been known from DE 39 02 402 C1.
Another testing element, in which the gas to be tested is drawn through an indicator strip, to has been known from DE 34 07 686 C1. The reagents on the indicator strip react specifically with the components of the sample to be determined. This leads to a change in color, which can be detected photometrically.
Another embodiment of such testing elements is that of a badge. A flat disk, e.g., a paper disk, impregnated with the reagent, reacts with the components of the air due to diffusion, while the color of the disk changes. The intensity of the change in color within a defined period of time is a measure of the concentration of the component present in the air.
The colorimetric measuring system used most frequently is based on redox reactions. A decisive requirement on the reagent system is the change in the intrinsic color with a sufficient color intensity during the reaction process. For example, the intrinsic color of the chromosulfuiric acid reagent system changes from yellow to green as a consequence of the reduction of chromium(VI) to chromium(III). Another example is the change in color from purple to yellowish brown in the permanganate system as a consequence of the reduction of Mn(VII) to Mn(IV).
However, the intensity of the intrinsic color is not very high in many reagent systems, or the maximum of the change in color is at a wavelength that can be read, e.g., optoelectronically, with difficulty only. Furthermore, there are a large number of reagent systems in which the intrinsic color changes only slightly if at all in the course of the reaction with the analyte, even though strong oxidizing agents are used.
Redox indicators, whose use depends on the change potential of the redox system, are used in titrimetric analysis in the liquid phase to better recognize or visualize the point of change. The examples of prior-art redox indicators include indigosulfonates, methylene blue, iodine-potassium iodide-starch, indophenols, diphenylbenzidine, triphenylmethane dyes, phenylanthranilic acid, Ferroin, and nitro-Ferroin.
However, most analytes (e.g., ethanol, gasoline hydrocarbons, carbon monoxide) require strong oxidizing agents, under the effect of which most of the known redox indicators are decomposed, so that these are not suitable for the purposes mentioned.
Thus, the basic object of the present invention is to provide testing elements whose indicator components are stable even in a strongly oxidizing matrix and, in particular, display an intense change in color intensity. Another object of the present invention is to provide redox indicators for this purpose, which are stable, in particular, even on prolonged storage in the oxidation matrix.
Testing elements containing a carrier material to which at least one oxidizing agent and at least one redox indicator are applied are provided according to the present invention to accomplish this object, the redox indicator being in the oxidized form.
The redox indicator is preferably a benzidine derivative according to the general formula 
in which the radicals R are identical or different and denote a hydrogen, alkyl, aryl, halogen, trifluoromethyl, cyano, nitro, dialkylamino, ester, sulfoxyl ester, allyloxy or aryloxy, X denotes a halogen, Y denotes a trifluoromethyl, cyano, nitro, dialkylamino, ester, sulfoxyl ester, alkyloxy or aryloxy, n is an integer ranging from zero to three, and m equals 5xe2x88x92n.
R preferably denotes a hydrogen, n equals 1, and m equals 4. X preferably denotes a fluorine and Y preferably denotes a trifluoromethyl, cyano or nitro, Y denoting trifluoromethyl being most preferred.
The particularly preferred compounds are:
N,Nxe2x80x2-Bis[2,3,5,6-tetrafluoro-4-(trifluoromethyl)-phenyl]-benzidine, 
xe2x80x83N,Nxe2x80x2-bis[2,3,5,6-tetrafluoro-4-cyanophenyl]-benzidine, 
xe2x80x83and N,Nxe2x80x2-bis[2,3,5,6-tetrafluoro-4-nitrophenyl]-benzidine, 
By selecting different substituents Y, which act as a donor or acceptor on the conjugated system, the change potential and/or the maximum of the absorption or remission wavelength can be influenced as desired. For example, the electron-attracting trifluoromethyl and nitro groups ensure a reduction in the change potential.
Other preferred indicators are phenylenediamine or indophenol derivatives. Phenylenediamine-immonium salts are particularly preferred among these. The most preferred of these compounds are: 
and 2,6-dichlorophenol indophenol as well as its salts 
The compounds that can be used according to the present invention have high stability in a strongly oxidizing matrix, which is especially advantageous for systems in which the indicator is in prolonged contact with the oxidizing reagents. As a result, the storage stability of such compounds is advantageously increased. Other advantages that arise from the use of the indicators according to the present invention are the lowering of the detection limit and the reduction in the necessary number of strokes in detector tubes, the possibility of using oxidizing agents that display only a slight change in color during the reaction process (this is possible because the indicators do not depend on the reagent, but depend only on the current redox potential), as well as the reduction in the necessary amount of toxic oxidizing agents, e.g., chromate.
The benzidine derivatives used according to the present invention surprisingly additionally have improved storage stability, especially under oxidative conditions, against all the other compounds mentioned.
The determination of the gas and/or vapor components of a gas mixture is preferably performed in detector tubes. The detector tubes comprise a glass tube, on the inner space of which fine-grained carrier material is fixed. For example, silica gel, quartz glass, aluminum oxide or plastic may be used as the carrier material. The oxidizing reagents and the redox indicator are applied in very thin layers to the carrier material.
It is also possible to apply a plurality of layers of different chemical reactivities within the same tube, e,g., in order to change an analyte in a preceding layer chemically such that it can be detected with a color reaction in a layer following that layer.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which a preferred embodiment of the invention is illustrated.